Bring me a shrubbery, one that looks nice and doesn’t extend the growing season
Summary
Invasive shrubs are an ongoing concern in eastern North American forests. Several invasive shrubs are known to have an extended leaf phenology, or earlier leaf-on plus later leaf-off, relative to native shrubs. Extended leaf phenology can provide competitive advantages to invasives and can negatively affect native understory species by increasing understory shade when the canopy would otherwise be leafless. However, prior to this study extended leaf phenology had only been measured at local scales, making it unclear how the phenology of invasive and native shrubs differs across the northeastern region. To study leaf phenology at the regional scale, Maynard-Bean et al. partnered with the USA National Phenology Network to record leaf-on and leaf-off of 14 species of native and invasive shrubs over a four-year period. Community scientists observed native shrubs including dogwood (Cornus spp.), spicebush (Lindera benzoin), and Viburnum spp. and invasive shrubs including barberry (Berberis thunbergii), burning bush (Euonymus alatus), honeysuckle (Lonicera spp.), and privet (Ligustrum spp.). With the exception of burning bush in the spring and barberry in the fall, invasive shrubs had consistently longer growing seasons than native shrubs across the region. However, the gap in timing between invasive and native species varied with latitude. At the southern end of the study region (N. Carolina, ~35N), invasive shrubs emerged ~38 days earlier and senesced ~39 days later. At the northern end of the study region (Maine, ~45N), invasive shrubs emerged ~15 days earlier and senesced ~22 days later. While year to year temperatures did not appear to have an effect on growing season differences between invasive and native plants, the regional pattern suggests that invasive shrubs are able to take advantage of warmer temperatures to further extend their growing seasons.
Take home points
Invasive shrubs have an extended leaf phenology (weeks to months longer) relative to native shrubs in the Northeast
Differences in extended leaf phenology between native and invasive species decrease the further north you go.
The timing of shrub leaf-on is generally more strongly correlated to climatic conditions than the timing of shrub leaf-off.
Management implications
Invasive shrubs are likely to continue to be easiest to detect in the early spring and late fall when native shrubs have no leaves.
Citizen science can be a viable strategy for recording phenology data on a large geographic scale. However, training and regular communication is needed to keep participants informed and motivated.
Keywords
shifting season, phenology, plant, terrestrial